Proton transfer aromatic substitution

The mechanism of the aromatic substitution may involve the attack of the dectrophilic NOj" " ion upon the nucleophilic aromatic nucleus to produce the carboniiim ion (I) the latter transfers a proton to the bisulphate ion, the most basic substance in the reaction mixture... 

Volume 8 Volume 9 Volume 10 Volume 12 Volume 13 Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)... 

Some theoretical aspects of thiophene reactivity and structure have also been discussed, for example the kinetics of proton transfer from 2,3-dihydrobenzo[6]thiophenc-2-onc <06JOC8203>, the configuration of imines derived from thiophenecarbaldehydes <06JOC7165>, and the relative stability of benzo[c]thiophene <06T12204>. The kinetics of nucleophilic aromatic substitution of some 2-substituted-5-nitrothiophenes in room temperature ionic liquids have also been investigated <06JOC5144>. 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

Previously, Ohashi and his co-workers reported the photosubstitution of 1,2,4,5-tetracyanobenzene (TCNB) with toluene via the excitation of the charge-transfer complex between TCNB and toluene [409], The formation of substitution product is explained by the proton transfer from the radical cation of toluene to the radical anion of TCNB followed by the radical coupling and the dehydrocyanation. This type of photosubstitution has been well investigated and a variety of examples are reported. Arnold reported the photoreaction of p-dicyanobenzene (p-DCB) with 2,3-dimethyl-2-butene in the presence of phenanthrene in acetonitrile to give l-(4-cyanophenyl)-2,3-dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyl-l-butene [410,411], The addition of methanol into this reaction system affords a methanol-incorporated product. This photoreaction was named the photo-NO-CAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) by Arnold. However, a large number of nucleophile-incorporated photoreactions have been reported as three-component addition reactions via photoinduced electron transfer [19,40,113,114,201,410-425], Some examples are shown in Scheme 120. 

A recent study of proton transfer from rhenium Fisher-type carbine complexes (13) shows that the reactions lead to the formation of an aromatic product (14), following the same rules as reactions that lead to the formation of products stabilized by simple resonance. The conjugate bases of these carbine complexes represent aromatic heterocycles, i.e., substituted furan, selenophene, and thiophene derivatives, respectively. The aromatic stabilization of these heterocycles is known to follow the order furan < selenophene < thiophene (Scheme 1) [43],... 

In several photochemical electron transfer reactions, addition products are observed between the donor and acceptor molecules. However, the formation of these products does not necessarily involve direct coupling of the radical ion pair. Instead, many of these reactions proceed via proton transfer from the radical cation to the radical anion, followed by coupling of the donor derived radical with an acceptor derived intermediate. For example, 1,4-dicyanobenzene and various other cyanoaromatic acceptors react with 2,3-dimethylbutene to give aromatic substitution products, most likely formed via an addition-elimination sequence [140]. 

The Friedel-Crafts alkylation and acylation are of very little, if any, synthetic interest when applied to heterocyclic aromatic bases the substitution of protonated heterocycles by nucleophilic carbon-centered radicals is instead successful. This reaction, because of the dominant polar effect which is mainly related to the charge-transfer character of the transition state (Scheme 1), reproduces most of the aspects of the Friedel-Crafts aromatic substitution, but reactivity and selectivity are the opposite. 

In the non-phenolic oxidative coupling reaction the electron-rich arene 19 undergoes electron transfer yielding the radical cation 20, which is preferably treated in chlorinated solvents or strongly acidic media. Attack of 20 on the electron-rich reaction partner 21 will proceed in the same way as an electrophilic aromatic substitution involving adduct 22 which extrudes a proton. The intermediate radical 23 is subsequently oxidized to the cationic species 24 which forms the biaryl 25 by rearomatization. In contrast with the mechanism outlined in Scheme 5, two different oxidation steps are required. 

These results clearly show that the potential energy surface can contain a series of minima. The fact that selectivity in re-attack by the F ions can be observed indicates that the differences between the energy barriers for the secondary reactions control the distribution of the final products. The multistep character of these processes is further illustrated by the reactions observed when enolate anions are used as reactant ions. The ambident enolate anions may react with methyl pentafluorophenyl ether at the carbon or the oxygen site. If they react with the carbon site at the fluorine-bearing carbon atoms, then the molecule in the F ion/molecule complex formed contains relatively acidic hydrogen atoms so that proton transfer to the displaced F ion may occur. An example is given in (47) where the enolate anion, generated by HF loss, is not observed. An intramolecular nucleophilic aromatic substitution occurs instead and leads to a second F ion/ molecule complex. The F" ion in this complex then re-attacks the substituted benzofuran molecule formed, either by proton transfer or SN2 substitution. 

This type of tautomerism occurs by a proton transfer and transforms a substituted azine, e.g., 170, into an isomer with exocyclic conjugation, e.g., 171. Transfer of a proton to a ring carbon atom, e.g., 172, is rarer, due to loss of aromaticity, but can occur in polyhydroxyazines (e.g., barbituric acid 173). Tautomerism in six-membered heterocycles has been reviewed <2006AHC(91)1>. Table 31 summarizes the tautomeric equilibria of monosubstituted azines and their benzo derivatives for dilute solutions in water at ca. 20 °C. 

CT interaction 132). In the case of substituted benzenes with benzylic hydrogens, proton transfer yields radicals. The main evidence for this CT photoreduction mechanism is the lack of any isotope effect on kt when toluene- -d3 is substituted for toluene-hg. In the absence of any labile protons on the aromatic half of the CT complex, such as with benzene itself, the CT interaction is primarily a quenching one, unless an external proton source is present, in which case the complex is apparently protonated 183>. 

The latter mechanism is met in amine-vinyl monomer systems [41-46] (see Scheme 4). Due to the small n-acceptor ability of normal substituted vinyl monomers, an interaction in the ground-state level does not take place. The exciplexes assumed are detectable in aromatic amine-acrylonitrile (AN) systems by their emission spectra, as is shown in Fig. 1 for typical examples. The emission bands at 350 nm (by JV,JV-dimethyl-p-toluidine (DMT)) and 370 nm (by p-phenylene diamine (TMPD) result from the normal fluorescence of the isolated amine. As can be seen, the intensity of the exciplex emission is much higher in the DMT-AN system. This corresponds to the higher polymerization efficiency of that system (<)>[, by A. = 313 nm and 80 K 0.6 for DMT 0.15 for TMPD [46]). Mainly, the much higher dipole moment of DMT (1.1 D) is responsible for this result. The cation radicals [46] or neutral radicals [42] of the amines formed after PET and proton transfer have been detected by ESR measurements. As expected, the rate of photopolymerization of the systems discussed increases with increasing... 

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